351C overheated


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Jul 3, 2010
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The OC (California)
My Car
31 Model A Station Wagon “Woody”
69 Mach 1 - 351C
72 Mach 1 - 351C
96 Cobra Convertible
Not my ‘72, but on my ‘69.  My son and I went to San Clemente to CarsNCoffee this morning.  I had noticed the radiator level was going low and haven’t figured out where the coolant is going.  It’s not in the oil, its not in the passenger carpet, and it wasn’t on the garage floor.  We made it to San Clemente and even idled for a while once we got there, about 25 miles.  On the way home, oh boy.  Not half way and I pulled off the highway quickly and looked for a nice sloped lot to park on and shut it down.  It puked its guts out.  We cooled it, refilled and continued.  We only made it halfway home from there and I was looking for another exit.  Eventually, we made it home, but it was hot.  I could hear the coolant boiling in the upper hose.  

I let it cool down and went to start it and try to figure out if it is the thermostat, the water pump, or a head gasket.  I didn’t really see bubbles upon start up and idle. As it got hotter, it appeared that coolant might be flowing—I couldn’t really tell, either that or it had bubbles.  Any good advice?  

I’ve been looking for an excuse for a stroker motor, but this is about 5 years before I was ready to do something like that. :classic_sad:


Do you have a good 4 core radiator and a fan shroud.?  I have known many people to have overheating Cleveland issues when they don’t have the fan shroud on 71 through 73s.  I know the 69s has a lot smaller fan shroud and a narrower radiator than the 71s.  

is your 69 an original Cleveland car, or was this an upgrade from a Windsor or 302?.

Let me know on the shroud thing, ok?


I have the big 24” radiator (an A/C car) with either 3 or 4 rows, can’t remember which.  The radiator is older, but I purchased it and know its history.  Generally the car has always run cool with that radiator.  It has a shrouded electric fan, and I am fairly certain the fan and shroud are not the issue here as the problem was bad moving (Freeway) or sitting.  This car was a 351W car since I swapped it in 1990.

Several things to look at. Does the hose to the water pump have a spring in it? If not sometimes they collapse from suction and then no water flow. If thermostat is opening then I would get and exhaust gas checker for the radiator. You have a clear container you put the test liquid in and run the engine if there is exhaust gas in the radiator the liquid changes color. Here is link to one type. https://www.napaonline.com/en/p/BK_7001006?cid=paidsearch_shopping_dcoe_google&campaign=GSC-Chemicals&campaign_id=6478876245&adgroup_id=115972171828&adtype=pla&gclid=Cj0KCQjwt4X8BRCPARIsABmcnOoojrQtVMahxxdYJuvJPfGp2jHcFukumdQCcULok5kpB6m7M1G2mp4aAqYFEALw_wcB

When you look in radiator do you see white deposits on the ends of the cooling tubes? If so you might be getting clogged up. You should only use distilled water mixed with your antifreeze. 

I would get and exhaust gas checker for the radiator. You have a clear container you put the test liquid in and run the engine if there is exhaust gas in the radiator the liquid changes color.  
Do you have to have antifreeze to do this?  I am thinking of getting another t-stat, drain the system, test the old one and new one.  Install whichever works, new if possible.  Then fill with water to test.  I don’t want to put antifreeze back in if it is going to just spit it back out.  The radiator appears in good condition looking through the cap.  But I want to pull it and flush everything while I am doing this.  I do have distilled water, but had to use tap to get home.

No you can have just water to do the gas check. If you have deposits in your radiator you can take radiator off and lay flat and use CLR Calcium Lime Remover to clean the deposits. You can get at grocery in cleaning area. 
If you get new thermostat get a 195 deg. and drill a 1/32" dia. hole out near the gasket area. This will let the air out of block as you fill the system. 

So I pulled the t-stat today, and it does open.  But it is possible it was sticking.  It had some rust/crud on it.  Major radiator flush, heater core flush, and attempted block flush.  Stuck it back in just to see how it ran.  It heated up and the water was flowing in the radiator.  I am going to do the combustion leak test next, it’s cheap.  You can rent the tools and buy a bottle of the test fluid for $10 at Autozone.  I figure I might as well test it.  I did buy a new t-stat, so don’t blast me for being cheap.  I actually thinking about throwing a new water pump on if it passes the combustion test.  

If It fails the combustion test, I’ve been looking at late model 5.0 swaps...  I know I was thinking stroker motor, but modern muscle has an appeal.  Get the torch!

Well—good news/bad news:  it passed the combustion chamber leak test.  Bad news because I don’t get a new stroker motor.  Good news because fixing with a head gasket leak is a lot of work.  Now to chase down the root cause.  The electric fans are working.  The thermostat is working (but may have stuck).  I have a new one but I am not going to install it until I get the system cleaned out.  There is lots of rust in the coolant—its nasty. I used a Prestone cooling system cleaner today and then flushed it....again...and again.  Planning to do it one more time, and again... Not sure if I should throw a new water pump at it while I am at it.  Should I be able to see the coolant “moving” through the radiator once the T-stat opens?  I could only detect the slightest of movement.  Thinking about pulling the radiator too and just taking it to be cleaned out.  Just do it all, do it once.  I did drive it about 20 miles today and while it did get warm, it did not overheat.

Taking the radiator in to get it tested (results will determine choice to repair or replace) should be top priority at this point.  Since you mentioned you have already observed considerable rust flakes / debris during your flushing there is a pretty good possibility the radiator tubes are restricted / blocked.

And yes, you should see a strong flow of coolant moving across the top of the radiator once the T-stat opens.  Once you have radiator cleaned / replaced, and  coolant is still not flowing, it suggests worn / eroded water pump impeller.  That would be the time to replace it.

A lot of rust often indicates someone used straight water for coolant. The rust in the coolant causes the impellers to erode at a faster rate. If you decide to replace the water pump check the timing cover plate for pitting and erosion.

I built it and it had antifreeze in it.  I guess the antifreeze gave up during its long nap.  But I am suspicious of the water pump impeller as well.

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When I rebuilt my 351 there were a number of rusty broken spring pieces in the block, from the spring coil in the radiator hose. From the score marks on the water pump impeller a couple of pieces had made it to the water pump. It took a fair bit of turning the bare block on an engine stand to finally get all the pieces out. Glad I found them before the final assembly as all the flushing, air blowing, etc during the initial cleaning of the block didn't flush them out.

Way back when, a co-worker's late 60s Bonneville was overheating in the summer, and he pulled the thermostat out thinking that would help. He would drain the antifreeze during the summer and fill it with water, thinking water cooled better. He asked me for some help on it. The tubes in the radiator didn't look bad, but didn't look like much flow through the radiator so I pulled the upper hose off the thermostat housing and had him start the engine. The water created a fountain maybe 6 inches high. After pulling the water pump and looking at the half size impeller blades I explained what was happening, impeller blades both rusting and wearing down, and a 50/50 antifreeze mixture would have prevented it, plus better cooling than straight water. After installing a new pump I showed him the difference, a fountain more than two feet high at idle. His car was 5 or 6 years old at the time. After flushing the radiator and installing a new thermostat and Prestone, no more overheating problems.

I've also encountered pressed on impellers turning on the shaft.